Nachos Instructional OS

CS 270, Tao Yang, Spring 2011

04/20/23 2

What is Nachos OS? Allow students to examine, modify and execute

operating system software. A skeletal OS that supports

kernel threads user-level processes Simulates MIPS instruction execution.

Running on a virtual machine, executed as a single Unix process in a hosting OS.

Over 9K lines of C++ code. Can understand its basic mechanisms by reading

about 1-2K lines of code.

System Layers

Base Operating System (Linux for our class)

Nachos kernel threads

Thread 1 Thread 2 Thread N

Nachos OS modules(Threads mgm, File System, Code execution/memory mapping,

System calls/Interrupt)

Simulated MIPS Machine(CPU, Memory, Disk, Console)

User processUser process

04/20/23 4

Steps to Install Nachos Obtain and install Nachos source code.

Copy the source code from ~cs270t/nachosSept20.tar.gz

Compile the source code using gmake Run threads demo under the threads

subdirectory (just run kernel test threads). Run user program demo under the userprog

subdirectory. http://www.cs.ucsb.edu/~cs270t/HW/war

mup.html

04/20/23 5

Nachos code directory

machine --- Basic machine specification (MIPS simulator).

threads --- threads management (HW1). userprog -- binary code execution and system

calls (HW2). vm -- virtual memory (HW3). filesys -- file system (HW3) test -- binary test code network -- networking protocol bin -- utilities/tools (binary format conversion)

04/20/23 6

Source code reading and HW1Objectives of next 2 weeks Scan through ~1,000-2,000 lines of code

under threads directory Learn how context switch is accomplished among

threads. Learn how thread scheduling is done. Learn how locks/synchronization are implemented

and used. Complete HW1

Few hundred lines of code Sample solution for Task 1, 2, &3 are available

04/20/23 7

Single-threaded vs multithreaded Process

main (){ Thread *t1 = new Thread("forked thread1"); Thread *t2 = new Thread("forked thread2");

t1->Fork(SimpleThread, 1); t2->Fork(SimpleThread, 2);

SimpleThread(3);}

SimpleThread(int i){ printf(“Hello %d\n”, i); currentThread->Yield();}

Sample Example of Nacho Threads

Start to fork and execute a function in each child thread.

Parent also executes the same function

Function executed by threads

Create 2 new threads.

04/20/23 9

Nachos Threads Nachos threads execute and share the same code, share same global

variables. Nachos scheduler maintains a ready list, containing all threads that are

ready to execute. Each thread is in one of four states: READY, RUNNING, BLOCKED,

JUST_CREATED. Each thread object maintains a context block. Thread object supports the following operations:

Thread(char *debugName). Create a thread. Fork(VoidFunctionPtr func, int arg). Let a thread execute a function. Yield(). Suspend the calling thread and select a new one for

execution. Sleep(). Suspend the current thread, change its state to BLOCKED,

and remove it from the ready list Finish()

Nachos Thread States and Transitions

running(user)

running(kernel)

readyblocked

Scheduler::Run

Scheduler::ReadyToRun

interrupt orexception

Thread::Sleep

Thread::Yield

Machine::Run,ExceptionHandler

When running in user mode, the thread executes within the machine simulator. HW 2 covers this.

In HW 1 we are only concerned with the states in this box.

04/20/23 11

Thread Switching

Switching involves suspending current thread, saving its state, and restoring the state of new thread.

Following code involved in execution: the old code, the new code, and the code that performs switching.

Switch(oldThread, newThread): Save all registers in oldThread's context block. Save the program address to be used when the old

thread is resumed. Load new values into the registers from the context

block of the new thread. Once the saved PC of the new thread is loaded,

Switch() is no longer executing.

04/20/23 12

Scheduler object for thread scheduling

A scheduler decides which thread to run next by scanning the ready list.

The scheduler is invoked whenever the current thread gives up the CPU.

The current Nachos scheduling policy is round-robin: new threads are appended to the end of the ready list, and the scheduler selects the front of the list.

The Scheduler object has the following operations: ReadyToRun(Thread *thread).Make thread ready to run and place it on the ready list. Thread *FindNextToRun() Run(Thread *nextThread)

04/20/23 13

Semaphore object for thread synchronization

Disable and re-enable interrupts to achieve mutual exclusion (e.g., by calling Interrupt::SetLevel()).

Operations for a Semaphore object: Semaphore(char* debugName, int initialValue) P(): Decrement the semaphore's count,

blocking the caller if the count is zero. V() :Increment the semaphore's count,

releasing one thread if any are blocked waiting on the count.

04/20/23 14

Key steps when Nachos executes

After you type ``nachos'' under threads subdirectory:

It is executing as a single Unix process. The main() calls

Initialize() to start up interrupt handling, create a scheduler for managing the ready queue.

ThreadTest() (to be explained for HW 1). currentThread->Finish() to let other threads

continue to run.

04/20/23 15

Key Calling graph when Nachos executes under thread directory

All files are in threads directory.

main() inmain.cc

Initialize()in system.cc

ThreadTest ()in threadtest.cc

Thread:Fork ()in thread.cc

Thread:Yield ()in thread.cc

StackAllocate()in thread.cc

FindNextToRun ()in scheduler.cc

ReadyToRun ()in scheduler.cc

Run ()in scheduler.cc

SWITCH ()in switch.s

ThreadRoot ()in switch.s

func() such as SimpleThread()in ThreadTest.cc

currentThread->Finish ()in threadtest.cc

04/20/23 16

ThreadRoot()

Executed by SWITCH(oldThread, nextThread)

jal StartupPC # call startup procedure. For a new thread, it is InterruptEnable().

move a0, InitialArg jal InitialPC # call main procedure jal WhenDonePC # when were done, call

clean up procedure.

04/20/23 17

QA

When will thread:Finish() be called? At the end of the forked thread. Check ThreadRoot

assembly code. At the end of the main() thread.

Thread:Finish() calls scheduler->run() to run a new thread. Will the old thread (that calls Thread:Finish()) be returned and continue to be executed?

No. Because the following is called first.threadToBeDestroyed = currentThread; Scheduler->Run() will delete such TCB

04/20/23 18

QA

In Sleep() or Yield(), the interrupt is turned off before calling scheduler->() to execute another thread.

When will interrupt be turned on?

In executing the switched thread, ThreadRoot() assembly code first executes StartupPC function which is

machineState[StartupPCState] = (int) InterruptEnable;

04/20/23 19

HW 1: threads & synchronization

Work under threads subdirectory. Modify ThreadTest() to do simple threads

programming (spawning multiple threads). Implement locks and condition variables

(missing from the file synch.cc). Workload:

Read Nachos code and add few hundred lines of code.

Undocumented sample solution is provided.

04/20/23 20

HW 1: Files involvedKey files main.cc, threadtest.cc -- a simple test of our thread routines. thread.h thread.cc -- Nachos thread data structure and operations. scheduler.h scheduler.cc -- The thread ready list. synch.h synch.cc -- synchronization routines.

Other related files. synchlist.h, synchlist.cc -- synchronized access to lists using

locks/conditions (useful examples for your programming). list.h list.cc -- generic list management. system.h, system.cc -- Nachos startup/shutdown routines. utility.h utility.cc -- some useful definitions and debugging routines. interrupt.h interrupt.cc -- manage interrupts. time.h timer.cc -- clock emulation. switch.h, switch.s -- assembly code for thread switching. stats.h stats.cc -- collect interesting statistics.

04/20/23 21

HW Sample solution~cs240t/sampleSolutionCode.tar.gz Has an old solution for HW1, HW2, HW3

HW1 threads subdirectory. ~400 lines of new code. ~50% are for Tasks 1/2/3. Code for task 4 is not useful.

HW2 -> userprog subdirectory. ~1300 lines of new code. HW3 -> vm and filesys. ~1200 lines of new code. 800 may be

enough.

Caveat: Mixed benefits/problems in using other students’ code.e.g. Not well documented, not fully tested. Possibly awkward

design. Still your responsibility to produce good solutions

(correctness, performance, style).